A bridge to nowhere: methane emissions and the greenhouse gas footprint of natural gas

PERSPECTIVE

A bridge to nowhere: methane emissions and thegreenhouse gas footprint of natural gasRobert W. Howarth

Department of Ecology & Evolutionary Biology, Cornell University, Ithaca, New York 14853

Keywords

Greenhouse gas footprint, methane

emissions, natural gas, shale gas

Correspondence

Robert W. Howarth, Department of Ecology

& Evolutionary Biology, Cornell University,

Ithaca, NY 14853. Tel: 607-255-6175;

E-mail: [email protected]

Funding Information

Funding was provided by Cornell University,

the Park Foundation, and the Wallace Global

Fund.

Received: 4 March 2014; Revised: 18 April

2014; Accepted: 22 April 2014

doi: 10.1002/ese3.35

Abstract

In April 2011, we published the first peer-reviewed analysis of the greenhouse

gas footprint (GHG) of shale gas, concluding that the climate impact of shale

gas may be worse than that of other fossil fuels such as coal and oil because of

methane emissions. We noted the poor quality of publicly available data to sup-

port our analysis and called for further research. Our paper spurred a large

increase in research and analysis, including several new studies that have better

measured methane emissions from natural gas systems. Here, I review this new

research in the context of our 2011 paper and the fifth assessment from the

Intergovernmental Panel on Climate Change released in 2013. The best data

available now indicate that our estimates of methane emission from both shale

gas and conventional natural gas were relatively robust. Using these new, best

available data and a 20-year time period for comparing the warming potential

of methane to carbon dioxide, the conclusion stands that both shale gas and

conventional natural gas have a larger GHG than do coal or oil, for any possi-

ble use of natural gas and particularly for the primary uses of residential and

commercial heating. The 20-year time period is appropriate because of the

urgent need to reduce methane emissions over the coming 15–35 years.

Introduction

Natural gas is often promoted as a bridge fuel that will

allow society to continue to use fossil energy over the

coming decades while emitting fewer greenhouse gases

than from using other fossil fuels such as coal and oil.

While it is true that less carbon dioxide is emitted per

unit energy released when burning natural gas compared

to coal or oil, natural gas is composed largely of methane,

which itself is an extremely potent greenhouse gas. Meth-

ane is far more effective at trapping heat in the atmo-

sphere than is carbon dioxide, and so even small rates of

methane emission can have a large influence on the

greenhouse gas footprints (GHGs) of natural gas use.

Increasingly in the United States, conventional sources

of natural gas are being depleted, and shale gas (natural gas

obtained from shale formations using high-volume hydrau-

lic fracturing and precision horizontal drilling) is rapidly

growing in importance: shale gas contributed only 3% of

United States natural gas production in 2005, rising to 35%

by 2012 and predicted to grow to almost 50% by 2035 [1].

The gas held in tight sandstone formations is another form

of unconventional gas, also increasingly obtained through

high-volume hydraulic fracturing and is growing in impor-

tance. In 2012, gas extracted from shale and tight-sands

combined made up 60% of total natural gas production,

and this is predicted to increase to 70% by 2035 [1]. To

date, shale gas has been almost entirely a North American

phenomenon, and largely a U.S. one, but many expect shale

gas to grow in global importance as well.

In 2009, I and two colleagues at Cornell University,

Renee Santoro and Tony Ingraffea, took on as a research

challenge the determination of the GHG of unconven-

tional gas, particularly shale gas, including emissions of

methane. At that time, there were no papers in the

peer-reviewed literature on this topic, and there were

ª 2014 The Author. Energy Science & Engineering published by the Society of Chemical Industry and John Wiley & Sons Ltd.

This is an open access article under the terms of the Creative Commons Attribution License, which permits use,

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1

relatively few papers even on the contribution of methane

to the GHG of conventional natural gas [2–4]. At the end

of 2009, the U.S. Environmental Protection Agency (EPA)

still did not distinguish between conventional gas and

shale gas, and they estimated methane emissions for the

natural gas industry using emission factors from a 1996

study conducted jointly with the industry [5]; shale gas is

not mentioned in that report, which is not surprising

since significant shale gas production only started in the

first decade of the 2000s.

We began giving public lectures on our analysis in

March 2010, and these attracted media attention. One of

our points was that it seemed likely that complete life

cycle methane emissions from shale gas (from well devel-

opment and hydraulic fracturing through delivery of gas

to consumers) were greater than from conventional natu-

ral gas. Another preliminary conclusion was that the EPA

methane emission estimates (as they were reported in

2009 and before, based on [5]) seemed at least two- to

three-fold too low. In response to public attention from

our lectures, the EPA began to reanalyze their methane

emissions [6], and in late 2010, EPA began to release

updated and far higher estimates of methane emissions

from the natural gas production segment [7]. In April

2011, we published our first paper on the role of methane

in the GHG of shale gas [8]. We concluded that (1) the

amount and quality of available data on methane emis-

sions from the natural gas industry were poor; (2) meth-

ane emissions from shale gas were likely 50% greater than

from conventional natural gas; and (3) these methane

emissions contributed significantly to a large GHG for

both shale gas and conventional gas, particularly when

analyzed over the timescale of 20-years following emis-

sion. At this shorter timescale – which is highly relevant

to the concept of natural gas as a bridge or transitional

fuel over the next two to three decades – shale gas

appeared to have the largest greenhouse warming conse-

quences of any fossil fuel (Fig. 1). Because our conclusion

ran counter to U.S. national energy policy and had large

implications for climate change, and because the underly-

ing data were limited and of poor quality, we stressed the

urgent need for better data on methane emissions from

natural gas systems. This need has since been amplified

by the Inspector General of the EPA [9].

Our paper received immense media coverage, as evi-

denced by Time Magazine naming two of the authors

0

15

30

45

60

75

Low Estimate High Estimate Low Estimate High Estimate Surface -mined Deep-Mined

Shale Gas Conventional Gas Coal Diesel Oil

Gra

ms

Carb

on p

er M

J

Methane

Indirect CO2

Direct CO2

0

15

30

45

60

75

Low Estimate High Estimate Low Estimate High Estimate Surface -mined Deep-Mined

Shale Gas Conventional Gas Coal Diesel Oil

Gra

ms

Carb

on p

er M

J

Methane

Indirect CO2

Direct CO2

Integrated 20-yr

Integrated 100-yr

Figure 1. Comparison of the greenhouse gas

footprint of shale gas, conventional natural

gas, coal, and oil to generate a given quantity

of heat. Two timescales for analyzing the

relative warming of methane and carbon

dioxide are considered: an integrated 20-year

period (top) and an integrated 100-year period

(bottom). For both shale gas and conventional

natural gas, estimates are shown for the low-

and high-end methane emission estimates

from Howarth et al. [8]. For coal, estimates are

given for surface-mined and deep-mined coal,

since methane emissions are greater for deeper

mines. Blue bars show the direct emissions of

carbon dioxide during combustion of the fuels;

the small red bars show the indirect carbon

dioxide emissions associated with developing

and using the fuels; and the magenta bars

show methane emissions converted to g C of

carbon dioxide equivalents using period-

appropriate global warming potentials.

Adapted from [8].

2 ª 2014 The Author. Energy Science & Engineering published by the Society of Chemical Industry and John Wiley & Sons Ltd.

Methane and Natural Gas R. W. Howarth

(Howarth and Ingraffea) “People who Mattered” to the

global news in the December 2011 Person of the Year

Issue [10]. The nine months after our paper was pub-

lished saw a flurry of other papers on the same topic, a

huge increase in the rate of publication on the topic of

methane and natural gas compared to prior years and

decades. While some of these offered support for our

analysis, most did not and were either directly critical of

our work, or without referring to our analysis reached

conclusions more favorable to shale gas as a bridge fuel.

Few of these papers published in the 9 months after our

April 2011 paper provided new data; many simply offered

different interpretations of previously presented informa-

tion (as is reviewed briefly below). However, in 2012 and

2013 many new studies were published with major new

insights and sources of data. In this paper, I briefly review

the work on methane and natural gas published between

April 2011 and February 2014, concentrating on those

studies that have produced new primary data.

There are four components that are central to evaluat-

ing the role of methane in the GHG footprint of natural

gas: (1) the amount of carbon dioxide that is directly

emitted as the fuel is burned and indirectly emitted to

obtain and use the fuel; (2) the rate of methane emission

from the natural gas system (often expressed as a fraction

of the lifetime production of the gas well, normalized to

the amount of methane in the gas produced); (3) the glo-

bal warming potential (GWP) of methane, which is the

relative effect of methane compared to carbon dioxide in

terms of its warming of the global climate system and is a

function of the time frame considered after the emission

of the methane; and (4) the efficiency of use of natural gas

in the energy system. The GHG is then determined as:

GHG footprint

¼½CO2emissionsþðGWP�methane emissionsÞ�=efficiency

There is widespread consensus on the magnitude of the

direct emissions of carbon dioxide, and the indirect emis-

sions of carbon dioxide used to obtain and use natural

gas (for example, in building and maintaining pipelines,

drilling and hydraulically fracturing wells, and compress-

ing gas), while uncertain, are also relatively small [8]. In

this paper, I separately consider each of the other three

factors (methane emissions, GWP, and efficiency of use)

in the context of our April 2011 paper [8] and the subse-

quent literature.

How Much Methane is Emitted byNatural Gas Systems?

We used a full life cycle analysis in our April 2011 paper,

estimating the amount of methane emitted to the atmo-

sphere as a percentage of the lifetime production of a gas

well (normalized to the methane content of the natural

gas), including venting and leakages at the well site but

also during storage, processing, and delivery to customers.

For conventional natural gas, we estimated a range of

methane emissions from 1.7% to 6% (mean = 3.8%), and

for shale gas a range of 3.6% to 7.9% (mean = 5.8%) [8].

We attributed the larger emissions from shale gas to vent-

ing of methane at the time that wells are completed, dur-

ing the flowback period after high-volume hydraulic

fracturing, consistent with the findings of the EPA 2010

report [7]. We assumed all other emissions were the same

for conventional and shale gas. We estimated that down-

stream emissions (emissions during storage, long-distance

transport of gas in high-pressure pipelines, and distribu-

tion to local customers) were 1.4–3.6% (mean = 2.5%) of

the lifetime production of a well, and that the upstream

emissions (at the well site and for gas processing) were in

the range of 0.3–2.4% (mean = 1.4%) for conventional

gas and 2.2–4.3% (mean = 3.3%) for shale gas (Table 1).

Table 1. Full life cycle-based methane emission estimates, expressed

as a percentage of total methane produced in natural gas systems,

separated by upstream emissions for conventional gas, upstream

emissions for unconventional gas including shale gas, and down-

stream emissions for all natural gas. Studies are listed chronologically,

and our April 2011 study is boldfaced.

Upstream

conventional

gas

Upstream

unconventional

gas Downstream

EPA 1996 [5] 0.2% – 0.9%

Hayhoe

et al. [2]

1.4 – 2.5

Jamarillo

et al. [4]

0.2 – 0.9

Howarth

et al. [8]

1.4 3.3 2.5

EPA [11] 1.6 3.0 0.9

Ventakesh

et al. [12]

1.8 – 0.4

Jiang et al. [13] – 2.0 0.4

Stephenson

et al. [14]

0.4 0.6 0.07

Hultman

et al. [15]

1.3 2.8 0.9

Burnham

et al. [16]

2.0 1.3 0.6

Cathles

et al. [17]

0.9 0.9 0.7

Total emissions are the sum of the upstream and downstream emis-

sions. Studies are listed chronologically by time of publication. Dashes

indicate no values provided. The full derivation of the estimates

shown here is provided elsewhere [18, 19].

ª 2014 The Author. Energy Science & Engineering published by the Society of Chemical Industry and John Wiley & Sons Ltd. 3

R. W. Howarth Methane and Natural Gas

Although there were no prior papers on methane emis-

sions from shale gas when our paper was published, we

can compare our estimates for conventional natural gas

with earlier literature (Table 1). Our mean estimates for

both upstream and downstream emissions were identical

to the “best estimate” of Hayhoe et al. [2], although that

paper presented a wider range of estimates for both

upstream and downstream. It is important to note that

we used several newer sources of information not avail-

able to Hayhoe et al. [2], making the agreement all the

more remarkable. The Howarth et al. [8] estimates were

substantially higher than the emission factors used by the

EPA through 2009 based on the 1996 joint EPA-industry

study [5], which were only 1.1% for total emissions, 0.2%

for upstream emissions, and 0.9% for downstream emis-

sions. In the only other peer-reviewed paper on life cycle

methane emissions from conventional gas published in

the decade or two before our paper, Jamarillo et al. [4]

relied on these same EPA emission factors, although new

data on downstream emissions had already shown these

emission factors to be too low [3].

Through late 2010 and the first half of 2011, the EPA

provided a series of updates on their methane emission

factors from the natural gas industry, giving estimates for

shale gas for the first time as well as substantially increas-

ing their estimates for conventional natural gas. These are

discussed in detail by us elsewhere [18, 19]. Note that the

EPA did not and still has not updated their estimates for

downstream emissions, still using a value of 0.9% from a

1996 study [5]. For upstream emissions, the revised EPA

estimates gave emission factors of 1.6% (an increase from

their earlier value of 0.2%) for conventional natural gas

and 3.0% for shale gas [18, 19]. Note that the EPA esti-

mates for upstream emissions presented in 2011 [11] were

14% higher than ours for conventional gas and 10%

lower than ours for shale gas. Total emissions were more

divergent, due to the large difference in downstream

emission estimates (Table 1).

In addition to the revised EPA emission factors, many

other papers presented life cycle assessments of methane

emissions from shale gas, conventional gas, or both in the

immediate 9 months after April 2011 (Table 1). We and

others have critiqued these publications in detail else-

where [18–20]. Here, I will emphasize four crucial points:

1 For the upstream emissions in Table 1, all studies

relied on the same type of poorly documented and

highly uncertain information. These poor-quality data

led us in Howarth et al. [8] to call for better measure-

ments on methane fluxes, conducted by independent

scientists. Several such studies have been published in

the past 2 years, as is discussed further below, and

these provide a more robust approach for estimating

methane emissions.

2 At least some of the differences among values in

Table 1 are due more to different assumptions about

the lifetime production of a shale gas well than to dif-

ferences in emissions per well [18, 20]. Note that the

upstream life cycle emissions are scaled to the lifetime

production of a well (normalized to the methane con-

tent of the gas produced for the estimates given in

Table 1), and this was very uncertain in 2011 since

shale gas development is such a new phenomenon [21].

A subsequent detailed analysis by the U.S. Geological

Survey has demonstrated that the mean lifetime pro-

duction of unconventional gas wells is in fact lower

than any of papers in Table 1 assumed [22], meaning

that upstream shale gas emissions per production of

the well from all of the studies should be higher, in

some cases substantially so [18, 20].

3 The downstream emissions in Table 1 are particularly

uncertain, as highlighted by both Hayhoe et al. [2] and

Howarth et al. [8]. Note that all of the other papers

listed in Table 1 base their downstream emissions on

the EPA emission factors from 1996 [5], and none are

higher than those EPA estimates, even though a 2005

paper in Nature demonstrated higher levels of emission

from long-distance pipelines in Europe [3]. Several of

the papers in Table 1 have downstream emissions that

are lower than the 1996 EPA values, as they are focused

on electric power plants and assume that these plants

are drawing on gas lines that have lower emissions than

the average, which would include highly leaky low-

pressure urban distribution lines [12–14, 16]. Some

recent papers have noted a high incidence of leaks in

natural gas distribution systems in two U.S. east coast

cities [23, 24], but these new studies have yet placed an

emission flux estimate on these leaks. Another study

demonstrated very high methane emissions from fossil

fuel sources in Los Angeles but could not distinguish

between downstream natural gas emissions and other

sources [25]. Given the age of gas pipelines and distri-

bution systems in the United States, it should come as

no surprise that leakage may be high [8, 18, 19]. Half

of the high-pressure pipelines in the United States are

older than 50 years [18], and parts of the distribution

systems in many northeastern cities consist of cast-iron

pipes laid down a century ago [24].

4 While one of the papers in Table 1 by Cathles and his

colleagues [17], characterized our methane emission

estimates as too high and “at odds with previous stud-

ies,” that in fact is not the case. As noted above, both

our downstream and upstream estimates for conven-

tional gas are in excellent agreement with one of the

few previous peer-reviewed studies [2]. Furthermore,

our upstream emissions are in good agreement with

the majority of the papers published in 9 months after



ours: for conventional gas, our mean estimate of 1.4%

compares with the mean for all the other studies in

Table 1 of 1.33%; if we exclude the very low estimate

from Stephenson et al. [14], which was based on an

analysis of what the gas industry is capable of doing

rather than on any new measurements, and also the

relatively low estimate from Cathles et al. [17], which

was based on the assumption that the gas industry

would not vent gas for economic and safety issues (see

critique of this in [18]), the mean of the other four

studies is 1.7, or almost twice as high as the Cathles

et al. [17] estimate and 20% higher than our estimate.

For shale gas, again excluding Stephenson et al. [14]

and Cathles et al. [17] as well as our estimate, the

other four studies in Table 1 have a mean estimate of

2.3, a value 2.5-fold greater than that from Cathles

et al. [17] and 30% less than our mean estimate. From

this perspective, the estimates of Cathles et al. [17]

appear to be greater outliers than are ours.

Since 2012, many new papers have produced additional

primary data (Fig. 2). Two of these found very high

upstream methane emission rates from unconventional

gas fields (relative to gross methane production), 4% for

a tight-sands field in Colorado [26] and 9% for a shale

gas field in Utah [27], while another found emissions

from a shale gas field in Pennsylvania to be broadly con-

sistent with the emission factors we had published in our

2011 paper [28]. All three of these studies inferred rates

from atmospheric data that integrated a large number of

wells at the basin scale. The new Utah data [27] are much

higher than any of the estimates previously published for

upstream emissions from unconventional gas fields

(Fig. 2), while the measurement for the Colorado tight-

sands field [26] overlaps with our high-end estimate for

upstream unconventional gas emissions in Howarth et al.

[8]. The Utah and Colorado studies may not be represen-

tative of the typical methane emissions for the entire Uni-

ted States, in part, because they focused on regions where

they expected high methane fluxes based on recent

declines in air quality. But I agree with the conclusion of

Brandt and his colleagues [29] that the “bottom-up” esti-

mation approaches that we and all the other papers in

Table 1 employed are inherently likely to lead to underes-

timates, in part, because some components of the natural

gas system are not included. As one example, the recent

Pennsylvania study, which quantified fluxes from discrete

locations on the ground by mapping methane plumes

from an airplane, found very high emissions from many

wells that were still being drilled, had not yet reached the

shale formation, and had not yet been hydraulically frac-

tured [28]. These wells represented only 1% of the wells

in the area but were responsible for 6–9% of the regional

methane flux from all sources. One explanation is that

the drill rigs encountered pockets of shallower gas and

released this to the atmosphere. We, the EPA, and all of

the papers in Table 1 had assumed little or no methane

emissions from wells during this drilling phase.

Allen and colleagues [30] published a comprehensive

study in 2013 of upstream emissions for both conven-

tional and unconventional gas wells for several regions in

the United States, using the same basic bottom-up

approach as the joint EPA-industry study of 1996 used

[5]. As with that earlier effort, this new study relied heav-

ily on industry cooperation, and was funded largely by

industry with coordination provided by the Environmen-

tal Defense Fund. For the United States as a whole at the

Howarth et al. 2011 conven onal gas

Petron et al. 2012ght-sands gas

Karion et al. 2012, shale gas

Allen et al. 2013, US average

Miller et al. 2013, US average

Brandt et al. 2013. US average

0% 2% 4% 6% 8%

Howarth et al. 2011 shale gas

4

9

3.6

7.1

0.42

total = 1.7total = 6

total = 3.6

total = 7.9

Upstream emissions

Downstream emissions

Total emissions (when not estimated separately)

low

high

3.6 or more

low

high

low

high

0.3 1.4

2.4 3.6 2.2 1.4

4.3 3.6

Figure 2. Comparison of recent new data on

methane emissions compared to the estimates

published in Howarth et al. [8]. Some of the

new data are for upstream emissions, while

others give only averages for natural gas

systems in the United States. No new

measurements for downstream emissions alone

have been published since 2005 [8, 26, 27, 29,

30, 32].



time of their study, Allen et al. [30] concluded that

upstream methane emissions were only 0.42% of the

natural gas production by the wells (Fig. 2), a value at

the low end of those seen in Table 1. Using the low-end

estimates, “best-case” scenarios for upstream emissions

from Howarth et al. [8] and the mix of shale gas and

conventional gas produced in the United States in 2012, I

estimate the U.S. national best-case emission rate would

be 0.5%, or similar to that observed by Allen and col-

leagues. It should not be surprising that their study, in

relying on industry access to their sampling points, ended

up in fact measuring the best possible performance by

industry.

In 2013, the EPA reduced their emission estimates for

the oil and gas industry, essentially halving their upstream

emissions for average natural gas systems from 1.8% to

0.88% for the year 2009 (with the mix of conventional

and unconventional gas for that year) from what they

had reported in 2011 and 2012; the EPA estimate for

downstream emissions remained at 0.9%, giving a total

national emission estimate of 1.8%. EPA took this action

to decrease their emission factors for upstream emissions

despite the publication in 2012 of the methane emissions

from a Colorado field [26] and oral presentations at the

American Geophysical Union meeting in December 2012

of the results subsequently published by Karion and col-

leagues [27] and Caulton and colleagues [28], all of which

would have suggested higher emissions, perhaps spectacu-

larly so. As is discussed by Karion et al. [27], the decrease

in the upstream methane emissions by EPA in 2013 was

driven by a non-peer-reviewed industry report [31] which

argued that emissions from liquid unloading and during

refracturing of unconventional wells were far lower than

used in the EPA [11] assessment. At least in part in

response to these changes by EPA, the Inspector General

for the EPA concluded that the agency needs improve-

ments in their approach to estimating emissions from the

natural gas industry [9].

An important paper published late in 2013 [32] indi-

cates the EPA made a mistake in reducing their emission

estimates earlier in the year. In this analysis, the most

comprehensive study to date of methane sources in the

United States, Miller and colleagues used atmospheric

methane monitoring data for 2007 and 2008 – 7710

observations from airplanes and 4984 from towers from

across North America – together with an inverse model

to assess total methane emissions nationally from all

sources. They concluded that rather than reducing meth-

ane emission terms between their 2011 and 2013 invento-

ries, EPA should have increased anthropogenic methane

emission estimates, particularly for the oil and gas indus-

try and for animal agriculture operations. They stated that

methane emissions from the United States oil and gas

industry are very likely two-fold greater or more than

indicated by the factors EPA released in 2013 [32]. This

suggests that total methane emissions from the natural

gas industry were at least 3.6% in 2007 and 2008 (Fig. 2).

In early 2014, Brandt and his colleagues [29] reviewed

the technical literature over the past 20 years on methane

emissions from natural gas systems. They concluded that

“official inventories consistently underestimate actual

methane emissions,” but also suggested that the very high

estimates from the top–down studies in Utah and Colo-

rado [26, 27] “are unlikely to be representative of typical

[natural gas] system leakage rates.” In the supplemental

materials for their paper, Brandt et al. [29] state that

methane emissions in the United States from the natural

gas industry are probably greater than the 1.8% assumed

by the EPA by an additional 1.8–5.4%, implying an aver-

age rate between 3.6% and 7.1% (mean = 5.4%) [33]

(Fig. 2).

This recent literature suggests to me that the emission

estimates we published in Howarth et al. [8] are surpris-

ingly robust, particularly for conventional natural gas

(Fig. 2). The results from two of the recent top–downstudies [26, 27] indicate our estimates for unconventional

gas may have been too low. Partly in response to our

work and their own reanalysis of methane emissions from

shale gas wells, EPA has now promulgated new regula-

tions that will as of January 2015 reduce methane emis-

sions at the time of well completions, requiring capture

and use of the gas instead in most cases. Some wells are

exempt, and the regulation does not apply to venting of

methane from oil wells, including shale oil wells, which

often have associated gas. Nonetheless, the regulations are

an important step in the right direction, and will certainly

help, if they can be adequately enforced. Even still,

though, results such as those from the Pennsylvania fly-

over showing high rates of methane emission during the

drilling phase of some shale gas wells [28] suggest that

methane emissions from shale gas may remain at levels

higher than from conventional natural gas.

The GWP of Methane

While methane is far more effective as a greenhouse gas

than carbon dioxide, methane has an atmospheric lifetime

of only 12 years or so, while carbon dioxide has an effec-

tive influence on atmospheric chemistry for a century or

longer [34]. The time frame over which we compare the

two gases is therefore critical, with methane becoming rel-

atively less important than carbon dioxide as the time-

scale increases. Of the major papers on methane and the

GHG for conventional natural gas published before our

analysis for shale gas, one modeled the relative radiative

forcing by methane compared to carbon dioxide continu-



ously over a 100-year time period following emission [2],

and two used the global warming approach (GWP) which

compares how much larger the integrated global warming

from a given mass of methane is over a specified period

of time compared to the same mass of carbon dioxide. Of

the two that used the GWP approach, one showed both

20-year and 100-year GWP analyses [3] while another

used only a 100-year GWP time frame [4]. Both used

GWP values from the Intergovernmental Panel on Cli-

mate Change (IPCC) synthesis report from 1996 [35], the

most reliable estimates at the time their papers were pub-

lished. In subsequent reports from the IPCC in 2007 [36]

and 2013 [34] and in a paper in Science by workers at the

NASA Goddard Space Institute [37], these GWP values

have been substantially increased, in part, to account for

the indirect effects of methane on other radiatively active

substances in the atmosphere such as ozone (Table 2).

In Howarth et al. [8], we used the GWP approach and

closely followed the work of Lelieveld and colleagues [3]

in presenting both integrated 20 and 100 year periods,

and in giving equal credence and interpretation to both

timescales. We upgraded the approach by using the most

recently published values for GWP at that time [37].

These more recent GWP values increased the relative

warming of methane compared to carbon dioxide by

1.9-fold for the 20-year time period (GWP of 105 vs. 56)

and by 1.6-fold for the 100-year time period (GWP of 33

vs. 21; Table 2). Our conclusion was that for the 20-year

time period, shale gas had a larger GHG than coal or oil

even at our low-end estimates for methane emission

(Fig. 1); conventional gas also had a larger GHG than

coal or oil at our mean or high-end methane emission

estimates, but not at the very low-end range for methane

emission (the best-case, low-emission scenario). At the

100-year timescale, the influence of methane was much

diminished, yet at our high-end methane emissions, the

GHG of both shale gas and conventional gas still

exceeded that of coal and oil (Fig. 1).

Of nine new reports on methane and natural gas pub-

lished in 9 months after our April 2011 paper [8], six

only considered the 100-year time frame for GWP, two

used both a 20- and 100-year time frame, and one used a

continuous modeling of radiative forcing over the 0–100time period (Table 2). Of the six papers that only exam-

ined the 100-year time frame, all used the lower GWP

value of 25 from the 2007 IPCC report rather than the

higher value of 33 published by Shindell and colleagues in

2009 that we had used; this higher value better accounts

for the indirect effects of methane on global warming.

Many of these six papers implied that the IPCC dictated

a focus on the 100-year time period, which is simply not

the case: the IPCC report from 2007 [36] presented both

20- and 100-year GWP values for methane. And two of

these six papers criticized our inclusion of the 20-year

time period as inappropriate [14, 17]. I strongly disagree

with this criticism. In the time since April 2011 I have

come increasingly to believe that it is essential to consider

the role of methane on timescales that are much shorter

than 100 years, in part, due to new science on methane

and global warming presented since then [34, 41, 42],

briefly summarized below.

The most recent synthesis report from the IPCC in

2013 on the physical science basis of global warming

highlights the role of methane in global warming at mul-

tiple timescales, using GWP values for 10 years in addi-

tion to 20 and 100 years (GWP of 108, 86, and 34,

respectively) in their analysis [34]. The report states that

“there is no scientific argument for selecting 100 years

compared with other choices,” and that “the choice of

time horizon . . .. depends on the relative weight assigned

to the effects at different times” [34]. The IPCC further

concludes that at the 10-year timescale, the current global

release of methane from all anthropogenic sources exceeds

(slightly) all anthropogenic carbon dioxide emissions as

agents of global warming; that is, methane emissions are

more important (slightly) than carbon dioxide emissions

Table 2. Comparison of the timescales considered in comparing the

global warming consequences of methane and carbon dioxide.

Publication

Timescale

considered

20-year

GWP

100-year

GWP

IPCC [35] 20 and 100 years 56 21

Hayhoe et al. [2] 0–100 years NA NA

Lelieveld et al. [3] 20 and 100 years 56 21

Jamarillo et al. [4] 100 years – 21

IPCC [36] 20 and 100 years 72 25

Shindell et al. [37] 20 and 100 years 105 33

Howarth et al. [8] 20 and 100 years 105 33

Hughes [20] 20 and 100 years 105 33

Venkatesh et al. [12] 100 years – 25

Jiang et al. [13] 100 years – 25

Wigley [38] 0–100 years NA NA

Stephenson et al. [14] 100 years – 25

Hultman et al. [15] 20 and 100 years 72, 105 25, 44

Skone et al. [39] 100 years – 25

Burnham et al. [16] 100 years – 25

Cathles et al. [17] 100 years – 25

Alvarez et al. [40] 0–100 years NA NA

IPCC [34] 10, 20, and 100 years 86 34

Brandt et al. [29] 100 years – 25

Studies are listed chronologically by time of publication. Values for

the global warming potentials at 20 and 100 years given, when used

in the studies. NA stands for not applicable and is shown when stud-

ies did not use the global warming potential approach. Dashes are

shown for studies that did not consider the 20-year GWP. Studies that

are bolded provided primary estimates on global warming potentials,

while other studies are consumers of this information.



for driving the current rate of global warming. At the 20-

year timescale, total global emissions of methane are

equivalent to over 80% of global carbon dioxide emis-

sions. And at the 100-year timescale, current global meth-

ane emissions are equivalent to slightly less than 30% of

carbon dioxide emissions [34] (Fig. 3).

This difference in the time sensitivity of the climate

system to methane and carbon dioxide is critical, and not

widely appreciated by the policy community and even

some climate scientists. While some note how the long-

term momentum of the climate system is driven by

carbon dioxide [15], the climate system is far more

immediately responsive to changes in methane (and other

short-lived radiatively active materials in the atmosphere,

such as black carbon) [41]. The model published in 2012

by Shindell and colleagues [41] and adopted by the Uni-

ted Nations [42] predicts that unless emissions of meth-

ane and black carbon are reduced immediately, the

Earth’s average surface temperature will warm by 1.5°Cby about 2030 and by 2.0°C by 2045 to 2050 whether or

not carbon dioxide emissions are reduced. Reducing

methane and black carbon emissions, even if carbon diox-

ide is not controlled, would significantly slow the rate of

global warming and postpone reaching the 1.5°C and

2.0°C marks by 15–20 years. Controlling carbon dioxide

as well as methane and black carbon emissions further

slows the rate of global warming after 2045, through at

least 2070 [41, 42] (Fig. 4).

Why should we care about this warming over the next

few decades? At temperatures of 1.5–2.0°C above the

1890–1910 baseline, the risk of a fundamental change in

the Earth’s climate system becomes much greater [41–43],possibly leading to runaway feedbacks and even more glo-

bal warming. Such a result would dwarf any possible ben-

efit from reductions in carbon dioxide emissions over the

next few decades (e.g., switching from coal to natural gas,

which does reduce carbon dioxide but also increases

methane emissions). One of many mechanisms for such

catastrophic change is the melting of methane clathrates

in the oceans or melting of permafrost in the Arctic.

Hansen and his colleagues [43, 44] have suggested that

warming of the Earth by 1.8°C may trigger a large and

rapid increase in the release of such methane. While there

is a wide range in both the magnitude and timing of pro-

jected carbon release from thawing permafrost and melt-

ing clathrates in the literature [45], warming consistently

leads to greater release. This release can in turn cause a

feedback of accelerated global warming [46].

To state the converse of the argument: the influence of

today’s emissions on global warming 200 or 300 years

into the future will largely reflect carbon dioxide, and not

Figure 3. Current global greenhouse gas emissions, as estimated by

the IPCC [34], weighted for three different global warming potentials

and expressed as carbon dioxide equivalents. At the 10-year time

frame, global methane emissions expressed as carbon dioxide

equivalents actually exceed the carbon dioxide emissions. Adapted

from [34].

Figure 4. Observed global mean temperature from 1900 to 2009

and projected future temperature under four scenarios, relative to the

mean temperature from 1890 to 1910. The scenarios include the

IPCC [36] reference, reducing carbon dioxide emissions but not other

greenhouse gases (“CO2 measures”), controlling methane, and black

carbon emissions but not carbon dioxide (“CH4 + BC measures”), and

reducing emissions of carbon dioxide, methane, and black carbon

(“CO2 + CH4 + BC measures”). An increase in the temperature to

1.5–2.0°C above the 1890–1910 baseline (illustrated by the yellow

bar) poses risk of passing a tipping point and moving the Earth into

an alternate state for the climate system. The lower bound of this

danger zone, 1.5° warming, is predicted to occur by 2030 unless

stringent controls on methane and black carbon emissions are

initiated immediately. Controlling methane and black carbon shows

more immediate results than controlling carbon dioxide emissions,

although controlling all greenhouse gas emissions is essential to

keeping the planet in a safe operating space for humanity. Adapted

from [42].



methane, unless the emissions of methane lead to tipping

points and a fundamental change in the climate system.

And that could happen as early as within the next two to

three decades.

An increasing body of science is developing rapidly that

emphasizes the need to consider methane’s influence over

the decadal timescale, and the need to reduce methane

emissions. Unfortunately, some recent guidance for life

cycle assessments specify only the 100-year time frame

[47, 48], and the EPA in 2014 still uses the GWP values

from the IPCC 1996 assessment and only considers the

100-year time period when assessing methane emissions

[49]. In doing so, they underestimate the global warming

significance of methane by 1.6-fold compared to more

recent values for the 100-year time frame and by four to

fivefold compared to the 10- to 20-year time frames [34,

37].

Climate Impacts of Different NaturalGas Uses

In Howarth et al. [8], we compared the greenhouse gas

emissions of shale gas and conventional natural gas to

those of coal and oil, all normalized to the same amount

of heat production (i.e., g C of carbon dioxide equivalents

per MJ of energy released in combustion). We also noted

that the specific comparisons will depend on how the

fuels are used, due to differences in efficiencies of use,

and briefly discussed the production of electricity from

coal versus shale gas as an example; electric-generating

plants on average use heat energy from burning natural

gas more efficiently than they do that from coal, and this

is important although not usually dominant in comparing

the GHGs of these fuels [8, 18–20]. We presented our

main conclusions in the context of the heat production

(Fig. 1), though, because evaluating the GHGs of the dif-

ferent fossil fuels for all of their major uses was beyond

the scope of our original study, and electricity production

is not the major use of natural gas. This larger goal of

separately evaluating the GHGs of all the major uses of

natural gas has not yet been taken on by other research

groups either.

In Figure 5 (left-hand panel), I present an updated

comparison of the GHGs of natural gas, diesel oil, and

coal based on the best available information at this time

(April 2014). Values are expressed as g C of carbon diox-

ide equivalents per MJ of energy released as in our 2011

paper [8] and Figure 1. The methane emissions in Fig-

ure 5 are the mean and range of estimates from the

recent review by Brandt and colleagues [29] (see Fig. 2),

normalized to carbon dioxide equivalents using the 20-

year mean GWP value of 86 from the latest IPCC assess-

ment [34]. As noted above, I believe the 20-year GWP is

an appropriate timescale, given the urgent need to control

methane emissions globally. Estimates for coal and diesel

oil are from our 2011 paper [8], using data for surface-

mined coal since that dominates the U.S. market [20].

The direct and indirect emissions of carbon dioxide are

combined and are the same values as in Howarth et al.

[8] and Figure 1. Direct carbon dioxide emissions follow

the High Heating Value convention [2, 8]. Clearly, using

the best available data on rates of methane emission [29],

natural gas has a very large GHG per unit of heat gener-

ated when considered at this 20-year timescale.

Of the studies listed in Tables 1 and 2 published after

our 2011 paper [8], most focused just on the comparison

of natural gas and coal to generate electricity, although

one also considered the use of natural gas as a long-dis-

tance transportation fuel [40]. For context, over the per-

iod 2008–2013 in the United States, 31% of natural gas

has been used to generate electricity and 0.1% as a trans-

portation fuel [50]. None of the studies listed in Tables 1

and 2, other than Howarth et al. [8], considered the use

of natural gas for its primary use: as a source of heat. In

the United States over the last 6 years, 32% of natural gas

60

30

0

140

70

0

Hea

t gen

erat

ion

(g C

car

bon

diox

ide

equi

vale

nts

per M

J)

Electricity production (g C carbon dioxide equivalents per M

J)

Nat

ural

gas

Nat

ural

gas

Die

sel o

il

Coal

Coal

Figure 5. Comparison of the greenhouse gas footprint for using

natural gas, diesel oil, and coal for generating primary heat (left) and

for using natural gas and coal for generating electricity (right). Direct

and indirect carbon dioxide emissions are shown in yellow and are

from Howarth et al. [8], while methane emissions shown as g C of

carbon dioxide equivalents using the 2013 IPCC 20-year GWP [34] are

shown in red. Methane emissions for natural gas are the mean and

range for the U.S. national average reported by Brandt and colleagues

[29] in their supplemental materials. Methane emissions for diesel oil

and for coal are from Howarth et al. [8] For the electricity production,

average U.S. efficiencies of 41.8% for gas and 32.8% for coal are

assumed [20]. Several studies present data on emissions for electricity

production in other units. One can convert from g C of CO2-

equivalents per MJ to g CO2-equivalents per kWh by multiplying by

13.2. One can convert from g C of CO2-equivalents per MJ to g C of

CO2-equivalents per kWh by multiplying by 3.6.



has been used for residential and commercial heating and

28% for industrial process energy [50]. The focus on

electricity is appropriate if the only question at hand is

“how does switching out coal for natural gas in the gener-

ation of electricity affect greenhouse gas emissions?”

However, policy approaches have pushed other uses of

natural gas – without any scientific support – as a way to

reduce greenhouse gas emissions, apparently on the mis-

taken belief that the analysis for electricity generation

applied to these other uses. Before exploring some of

these other uses of natural gas, I would like to further

explore the question of electricity generation.

Many of the papers listed in Tables 1 and 2 concluded

that switching from coal to natural gas for generating

electricity has a positive influence on greenhouse gas

emissions. Note, though, that for almost all of these

papers, the conclusion was driven by a focus on only the

100-year timescale [4, 12–14, 16, 17, 29, 39], on a very

low assumed level of methane emission [4, 12–14, 17,

39], or both. The differences in efficiency of use in elec-

tric power plants, comparing either current average plants

or best possible technologies, are relatively small com-

pared to the influence of the GWP on the calculation [8,

18, 20, 40]. Using a 20-year GWP framework and the

methane emission estimates from Howarth et al. [8], the

GHG from generating electricity with natural gas is larger

than that from coal [8, 18–20]. Alvarez and colleagues

[40] concluded that for electricity generation, the GHG of

using natural gas was less than for coal for all time frames

only if the rate of methane leakage was less than 3.2%.

Their analysis used the estimates for the radiative forcing

of methane from the IPCC 2007 synthesis [36], and if we

correct their estimate for the data in the 2013 IPCC

assessment [34], this “break-even point” becomes 2.8%. If

we further consider the uncertainty in the radiative forc-

ing of methane of 30% or more [34], this “break-even”

value becomes a range of 2.4–3.2%.

In Figure 5 (right-hand panel), I compare the GHGs of

natural gas and coal when used to generate electricity,

again using the High Heating Value convention [2, 8],

the latest IPCC value for the 20-year GWP [34] and the

range of methane emission estimates reported by Brandt

and colleagues [29]. No distinction is made for less

downstream emissions for the pipelines that feed electric

power plants, as is assumed in several other studies [12–14, 16], simply because no data exist with which to tease

apart downstream emissions specific for electric power

generation [51]. This analysis uses the average efficiency

for electric power plants currently operating in the United

States, 41.8% for gas and 32.8% for coal [20]. The emis-

sions per unit of energy produced as electricity are higher

than for the heat generation alone, due to these correc-

tions for efficiency. Although the difference in the foot-

prints for using the two fuels is less for the electricity

comparison than for the comparison for heat generation,

at this 20-year timescale the GHG of natural gas remains

greater than that of coal, even at the low-end methane

emission estimate. This conclusion still holds when one

compares the fuels using the best available technologies

(50.2% efficiency for natural gas and 43.3% for coal

[20]); the emissions per unit of electricity generated

decrease for both by approximately the same amount.

For the dominant use of natural gas – heating for

water, domestic and commercial space, and industrial

process energy – the analysis we presented in our 2011

paper [8] and shown in Figure 1 remains the only pub-

lished study before this new analysis shown in Figure 5

(left-hand panel). The updated version shown here com-

pellingly indicates natural gas is not a climate-friendly

fuel for these uses. However, the greenhouse gas conse-

quences may in fact be worse than Figure 5 or Howarth

et al. [8] indicate, as I discuss next.

A recent study supported by the American Gas Foun-

dation promoted the in-home use of natural gas over

electricity for appliances (domestic hot water, cooking)

because of a supposed benefit for greenhouse gas emis-

sions [52]. The report argues that an in-home natural gas

appliance will have a higher efficiency in using the fuel

(up to 92%) compared to the overall efficiency of pro-

ducing and using electricity (“only about 40%,” according

to this study). However, they did not include methane

emissions in their analysis, nor did they consider the

extremely high efficiencies available for some electrical

appliances, such as in-home air-sourced heat pumps for

domestic hot water. For a given input of electricity, such

heat pumps can produce 2.2-times more heat energy,

since they are harvesting and concentrating heat from the

local environment [53]. In a comparison of using in-

home gas-fired water heaters or in-home high-efficiency

electric heat pumps, with the electricity for the heat

pumps generated by burning coal, the heat pumps had a

lower GHG than did in-home use of gas if the emission

rate for methane was greater than 0.7% for a 20-year

GWP or 1.3% for a 100-year GWP [51]. Using the mean

methane emission estimate from Howarth et al. [8] for

conventional natural gas (Fig. 2) and a 20-year GWP, the

in-home natural gas heater had a GHG that was twice

as large as that of the heat pump [51]. Of course, an

in-home heat pump powered by electricity from renew-

able sources such as wind and solar would have a far

smaller GHG yet [54].

What about other uses of natural gas? The “Natural

Gas Act,” a bill introduced in the United States Congress

in 2011 with bipartisan support and the backing of Presi-

dent Obama, would have provided tax subsidies to

encourage the replacement of diesel fuel by natural gas



for long-distance trucks and buses; the bill did not pass,

in part because conservatives opposed it as “market

distorting” [55, 56]. In Quebec, industry has claimed that

this replacement of diesel by shale gas would reduce

greenhouse gas emissions by up to 30% [57]. However, in

contrast to a possible advantage in replacing coal with

natural gas for electricity generation (if methane emis-

sions can be kept low enough), using natural gas to

replace diesel fuel as a long-distance transportation fuel

would greatly increase greenhouse emissions [29, 40]. In

part, this is because the energy of natural gas is used with

less efficiency than diesel in truck engines. Furthermore,

although methane emissions from transportation systems

have not been well measured, one could imagine signifi-

cant emissions during refueling operations for buses and

trucks, as well as from venting of on-vehicle natural gas

tanks to keep gas pressures significantly safe during warm

weather. Despite the findings of Alvarez and colleagues

published in 2012 [40], the EPA continues to indicate

that switching buses from diesel fuel to natural gas

reduces greenhouse gas emissions [58].

Concluding Thoughts

By 1950, which is about the time I was born, human

activity had contributed enough greenhouse gases to the

atmosphere to cause a radiative forcing – the driving fac-

tor behind global warming – of 0.57 watts m�2 compared

to before the industrial revolution [34]. Thirty years later,

in 1980 when I taught my first course on the biosphere

and global change, this human influence had doubled the

anthropogenic radiative forcing, to 1.25 watts m�2 [34].

And another 30 years later, the continued release of

greenhouse gases by humans has again doubled the forc-

ing, now at 2.29 watts m�2 or fourfold greater than just

60 years ago [34]. The temperature of the Earth continues

to rise in response at an alarming rate, and the climate

scientists tell us we may reach dangerous tipping points

in the climate system within just a few decades [34, 41,

42]. Is it too late to begin a serious reduction in green-

house gas emissions? I sincerely hope not, although surely

society has been very slow to respond to this risk. The

use of fossil fuels is the major cause of greenhouse gas

emissions, and any genuine effort to reduce emissions

must begin with fossil fuels.

Is natural gas a bridge fuel? At best, using natural gas

rather than coal to generate electricity might result in a

very modest reduction in total greenhouse gas emissions,

if those emissions can be kept below a range of 2.4–3.2%(based on [40], adjusted for the latest information on

radiative forcing of methane [34]). That is a big “if,” and

one that will require unprecedented investment in natural

gas infrastructure and regulatory oversight. For any other

foreseeable use of natural gas (heating, transportation),

the GHG is larger than if society chooses other fossil

fuels, even with the most stringent possible control on

methane emissions, if we view the consequences through

the decadal GWP frame. Given the sensitivity of the glo-

bal climate system to methane [41, 42], why take any risk

with continuing to use natural gas at all? The current role

of methane in global warming is large, contributing

1.0 watts m�2 out of the net total 2.29 watts m�2 of radi-

ative forcing [34].

Am I recommending that we continue to use coal and

oil, rather than replace these with natural gas? Not at all.

Society needs to wean itself from the addiction to fossil

fuels as quickly as possible. But to replace some fossil

fuels (coal, oil) with another (natural gas) will not suffice

as an approach to take on global warming. Rather, we

should embrace the technologies of the 21st Century, and

convert our energy systems to ones that rely on wind,

solar, and water power [59, 60, 61]. In Jacobson et al.

[54], we lay out a plan for doing this for the entire state

of New York, making the state largely free of fossil fuels

by 2030 and completely free by 2050. The plan relies only

on technologies that are commercially available at present,

and includes modern technologies such as high-efficiency

heat pumps for domestic water and space heating. We

estimated the cost of the plan over the time frame of

implementation as less than the present cost to the resi-

dents of New York from death and disease from fossil

fuel caused air pollution [54]. Only through such techno-

logical conversions can society truly address global

change. Natural gas is a bridge to nowhere.

Acknowledgments

Funding was provided by Cornell University, the Park

Foundation, and the Wallace Global Fund. I thank Bon-

gghi Hong, Roxanne Marino, Tony Ingraffea, George

Woodwell, and two reviewers who have asked to remain

anonymous for their valuable comments on earlier drafts

of the manuscript.

Conflict of Interest

None declared.

References

1. EIA. 2013. Annual energy outlook 2013 early release.

Energy Information Agency, US Department of Energy.

Available at http://www.eia.gov/energy_in_brief/article/

about_shale_gas.cfm (accessed 27 December 2013).

2. Hayhoe, K., H. S. Kheshgi, A. K. Jain, and D. J. Wuebbles.

2002. Substitution of natural gas for coal: climatic effects

of utility sector emissions. Clim. Change 54:107–139.



3. Lelieveld, J., S. Lechtenbohmer, S. S. Assonov, C. A. M.

Brenninkmeijer, C. Dinest, M. Fischedick, et al. 2005.

Low methane leakage from gas pipelines. Nature 434:

841–842.

4. Jamarillo, P., W. M. Griffin, and H. S. Mathews. 2007.

Comparative life-cycle air emissions of coal, domestic

natural gas, LNG, and SNG for electricity generation.

Environ. Sci. Technol. 41:6290–6296.

5. Harrison, M. R., T. M. Shires, J. K. Wessels, and R. M.

Cowgill. 1996. Methane emissions from the natural gas

industry. Volume 1: executive summary. EPA-600/

R-96-080a. U.S. Environmental Protection Agency, Office

of Research and Development, Washington, DC.

6. Personal communication from Roger Fernandez, US EPA.

19 May 2011.

7. EPA. 2010. Greenhouse gas emissions reporting from the

petroleum and natural gas industry. Background technical

support document. U.S. Environmental Protection Agency,

Washington, DC. Available at http://www.epa.gov/

climatechange/emissions/downloads10/Subpart-W_TSD.pdf

(accessed 24 February 2011).

8. Howarth, R. W., R. Santoro, and A. Ingraffea. 2011.

Methane and the greenhouse gas footprint of natural

gas from shale formations. Clim. Change Lett. 106:679–

690. doi: 10.1007/s10584-011-0061-5

9. U.S. Environmental Protection Agency Office of Inspector

General. 2013. EPA needs to improve air emissions data

for the oil and natural gas production sector. EPA OIG,

Washington, DC.

10. Walsh, B. 2011. People who mattered: Mark Ruffalo,

Anthony Ingraffea, Robert Howarth. Time, Person of the

Year issue on line, 14 December 2011. Available at http://

content.time.com/time/specials/packages/article/

0,28804,2101745_2102309_2102323,00.html (accessed 30

December 2011).

11. EPA. 2011. Inventory of U.S. greenhouse gas emissions

and sinks: 1990–2009. 14 April 2011. U.S. Environmental

Protection Agency, Washington, DC. Available at http://

epa.gov/climatechange/emissions/usinventoryreport.html

(accessed 25 November 2011).

12. Venkatesh, A., P. Jamarillo, W. M. Griffin, and H. S.

Matthews. 2011. Uncertainty in life cycle greenhouse gas

emissions from United States natural gas end-uses and its

effect on policy. Environ. Sci. Technol. 45:8182–8189.

13. Jiang, M., W. M. Griffin, C. Hendrickson, P. Jaramillo,

J. van Briesen, and A. Benkatesh. 2011. Life cycle

greenhouse gas emissions of Marcellus shale gas. Environ.

Res. Lett. 6:034014. doi: 10.1088/1748-9326/6/3/034014

14. Stephenson, T., J. E. Valle, and X. Riera-Palou. 2011.

Modeling the relative GHG emissions of conventional and

shale gas production. Environ. Sci. Technol. 45:10757–

10764.

15. Hultman, N., D. Rebois, M. Scholten, and C. Ramig. 2011.

The greenhouse impact of unconventional gas for

electricity generation. Environ. Res. Lett. 6:044008. doi: 10.

1088/1748-9326/6/4/044008

16. Burnham, A., J. Han, C. E. Clark, M. Wang, J. B. Dunn,

and I. P. Rivera. 2011. Life-cycle greenhouse gas emissions

of shale gas, natural gas, coal, and petroleum. Environ. Sci.

Technol. 46:619–627.

17. Cathles, L. M., L. Brown, M. Taam, and A. Hunter. 2012.

A commentary on “The greenhouse-gas footprint of

natural gas in shale formations” by R.W. Howarth, R.

Santoro, and Anthony Ingraffea. Clim. Change 113:525–

535.

18. Howarth, R. W., R. Santoro, A. Ingraffea. Venting and

leakage of methane from shale gas development: reply to

Cathles et al. 2012. Clim. Change 113:537–549. doi: 10.

1007/s10584-012-0401-0

19. Howarth, R. W., D. Shindell, R. Santoro, A. Ingraffea, N.

Phillips, and A. Townsend-Small. 2012. Methane

emissions from natural gas systems. Background paper

prepared for the National Climate Assessment, Reference

# 2011-003, Office of Science & Technology Policy

Assessment, Washington, DC. Available at http://www.

eeb.cornell.edu/howarth/Howarth%20et%20al.%20–%

20National%20Climate%20Assessment.pdf (accessed 1

March 2012).

20. Hughes, D. 2011. Lifecycle greenhouse gas emissions

from shale gas compared to coal: an analysis of two

conflicting studies. Post Carbon Institute, Santa Rosa, CA.

Available at http://www.postcarbon.org/reports/

PCI-Hughes-NETL-Cornell-Comparison.pdf (accessed 30

October 2011).

21. Howarth, R. W., and A. Ingraffea. 2011. Should fracking

stop? Yes, it is too high risk. Nature 477:271–273.

22. USGS. 2012. Variability of distributions of well-scale

estimated ultimate recovery for continuous

(unconventional) oil and gas resources in the United

States. U.S. Geological Survey, USGS Open-File Report

2012–1118. Available at http://pubs.usgs.gov/of/2012/1118/

(accessed 5 January 2014).

23. Phillips, N. G., R. Ackley, E. R. Crosson, A. Down, L.

Hutyra, M. Brondfield, et al. 2013. Mapping urban

pipeline leaks: methane leaks across Boston. Environ.

Pollut. 173:1–4.

24. Jackson, R. B., A. Down, N. G. Phillips, R. C. Ackley, C.

W. Cook, D. L. Plata, et al. 2014. Natural gas pipeline

leaks across Washington, DC. Environ. Sci. Technol.

48:2051–2058.

25. Townsend-Small, A., S. C. Tyler, D. E. Pataki, X. Xu, and

L. E. Christensen. 2012. Isotopic measurements of

atmospheric methane in Los Angeles, California, USA

reveal the influence of “fugitive” fossil fuel emissions.

J. Geophys. Res. 117:D07308.

26. P�etron, G., G. Frost, B. T. Miller, A. I. Hirsch, S. A.

Montzka, A. Karion, et al. 2012. Hydrocarbon emissions

characterization in the Colorado Front Range – a pilot



study. J. Geophys. Res. 117:D04304. doi: 10.1029/

2011JD016360

27. Karion, A., C. Sweeney, G. P�etron, G. Frost, R. M.

Hardesty, J. Kofler, et al. 2013. Methane emissions

estimate from airborne measurements over a western

United States natural gas field. Geophys. Res. Lett.

40:4393–4397.

28. Caulton, D. R., P. B. Shepson, R. L. Santoro, J. P. Sparks,

R. W. Howarth, A. Ingaffea, et al. 2014. Toward a better

understanding and quantification of methane emissions

from shale gas development. Proc. Natl. Acad. Sci. USA

111:6237–6242.

29. Brandt, A. F., G. A. Heath, E. A. Kort, F. O. O’Sullivan,

G. P�etron, S. M. Jordaan, et al. 2014. Methane leaks

from North American natural gas systems. Science

343:733–735.

30. Allen, D. T., V. M. Torres, K. Thomas, D. W. Sullivan, M.

Harrison, A. Hendler, et al. 2013. Measurements of

methane emissions at natural gas production sites in the

United States. Proc. Natl. Acad. Sci. USA 110:17768–

17773.

31. Shires, T., and M. Lev-On 2012. P. 48 in Characterizing

pivotal sources of methane emissions from unconventional

natural gas production: summary and analysis of API and

ANGA survey responses. American Petroleum Institute,

American Natural Gas Alliance, Washington, DC.

32. Miller, S. M., S. C. Wofsy, A. M. Michalak, E. A. Kort, A.

E. Andrews, S. C. Biraud, et al. 2013. Anthropogenic

emissions of methane in the United States. Proc. Natl.

Acad. Sci. USA 110:20018–20022.

33. Romm, J. 2014. By the time natural gas has a net climate

benefit, you’ll likely be dead and the climate ruined.

Climate Progress, 19 February 2014. Available at http://

thinkprogress.org/climate/2014/02/19/3296831/

natural-gas-climate-benefit/# (accessed 2 March 2014).

34. IPCC. 2013. Climate change 2013: the physical science

basis. Intergovernmental Panel on Climate Change.

Available at https://www.ipcc.ch/report/ar5/wg1/ (accessed

10 January 2014).

35. IPCC. 1996. IPCC second assessment, climate change,

1995. Intergovernmental Panel on Climate Change.

Available at http://www.ipcc.ch/pdf/climate-changes-1995/

ipcc-2nd-assessment/2nd-assessment-en.pdf (accessed 22

February 2014).

36. IPCC. 2007. IPCC Fourth Assessment Report (AR4),

Working Group 1, the physical science basis.

Intergovernmental Panel on Climate Change. Available at

http://www.ipcc.ch/publications_and_data/ar4/wg1/en/

contents.html (accessed 22 February 2014).

37. Shindell, D. T., G. Faluvegi, D. M. Koch, G. A. Schmidt,

N. Unger, and S. E. Bauer. 2009. Improved attribution of

climate forcing to emissions. Science 326:716–718.

38. Wigley, T. M. L. 2011. Coal to gas: the influence of

methane leakage. Clim. Change Lett. 108:601–608.

39. Skone, T. J., J. Littlefield, and J. Marriott. 2011. Life cycle

greenhouse gas inventory of natural gas extraction,

delivery and electricity production. Final report 24

October 2011 (DOE/NETL-2011/1522). U.S. Department

of Energy, National Energy Technology Laboratory,

Pittsburgh, PA.

40. Alvarez, R. A., S. W. Pacala, J. J. Winebrake, W. L.

Chameides, and S. P. Hamburg. 2012. Greater focus

needed on methane leakage from natural gas

infrastructure. Proc. Natl. Acad. Sci. USA 109:6435–6440.

doi: 10.1073/pnas.1202407109

41. Shindell, D., J. C. I. Kuylenstierna, E. Vignati, R. van

Dingenen, M. Amann, Z. Klimont, et al. 2012.

Simultaneously mitigating near-term climate change and

improving human health and food security. Science

335:183–189.

42. UNEP/WMO. 2011. Integrated assessment of black carbon

and tropospheric ozone: summary for decision makers.

United Nations Environment Programme and the World

Meteorological Organization, Nairobi, Kenya.

43. Hansen, J., M. Sato, P. Kharecha, G. Russell, D. W. Lea,

and M. Siddall. 2007. Climate change and trace gases.

Philos. Trans. R. Soc. A 365:1925–1954.

44. Hansen, J., and M. Sato. 2004. Greenhouse gas growth

rates. Proc. Natl. Acad. Sci. USA 101:16109–16114.

45. Schaefer, K., T. Zhang, L. Bruhwiler, and A. Barrett.

2011. Amount and timing of permafrost carbon release

in response to climate warming. Tellus 63:165–180.

doi: 10.1111/j.1600-0889.2011.00527.x

46. Zimov, S. A., E. A. G. Schuur, and F. S. Chapin. 2006.

Permafrost and the global carbon budget. Science

312:1612–1613.

47. BSI. 2011. Specification for the assessment of the life cycle

greenhouse gas emissions of goods and services. British

Standards Institute, Lond.

48. WRI/WBCSD. 2012. Product life cycle accounting and

reporting standard. World Resources Institute,

Washington, DC.

49. EPA. 2014. Overview of greenhouse gases. US

Environmental Protection Agency. Available at http://epa.

gov/climatechange/ghgemissions/gases/ch4.html (accessed

17 February 2014).

50. EIA. 2014. Natural gas consumption by end use. Energy

Information Agency, US Department of Energy. Available

at http://www.eia.gov/dnav/ng/ng_cons_sum_dcu_nus_a.

htm (accessed 3 March 2014).

51. Hong, B., and R. W. Howarth. In review. Assessing an

acceptable level of methane emissions from using natural

gas: domestic hot water example.

52. IHS CERA. 2014. Fueling the future with natural gas:

bringing it home. Executive Summary. January 2014.

Available at www.fuelingthefuture.org/assets/content/

AGF-Fueling-the-Future-Study.pdf (accessed 2 March

2014).



53. American Council for and Energy-Efficient Economy.

2014. Water heating. Available at http://www.aceee.org/

consumer/water-heating (accessed 3 February 2014).

54. Jacobson, M. Z., R. W. Howarth, M. A. Delucchi, S. R.

Scobies, J. M. Barth, M. J. Dvorak, et al. 2013. Examining

the feasibility of converting New York State’s all-purpose

energy infrastructure to one using wind, water, and

sunlight. Energy Policy 57:585–601.

55. Weis, D. J., and S. Boss. 2011. Conservatives power big oil,

stall cleaner natural gas vehicles. Center for American

Progress, 6 June 2011. Available at http://www.

americanprogress.org/issues/2011/06/nat_gas_statements.

html (accessed 2 March 2014).

56. Dolan, E. 2013. What stands in the way of natural gas

replacing gasoline in the US? OilPrice.com, 8 January

2013. Available at http://oilprice.com/Energy/Natural-Gas/

What-Stands-in-the-Way-of-Natural-Gas-Replacing-

Gasoline-in-the-US.html (accessed 2 March 2014).

57. Beaudine, M. 2010. In depth: shale gas exploration in

Quebec. The Gazette, 15 November 2010.

58. EPA. 2014. Sources of greenhouse gas emissions. US

Environmental Protection Agency. Available at http://www.

epa.gov/climatechange/ghgemissions/sources/

transportation.html (accessed 21 February 2014).

59. Jacobson, M. Z. 2009. Review of solutions to global

warming, air pollution, and energy security. Energy

Environ. Sci. 2:148–173.

60. Jacobson, M. A., and M. A. Delucchi. 2009. A path to

sustainable energy by 2030. Scientific American, November

2009.

61. Jacobson, M. A., and M. A. Delucchi. 2011. Providing all

global energy with wind, water, and solar power, Part I:

technologies, energy resources, quantities and areas

of infrastructure, and materials. Energy Policy 39:

1154–1169.



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A bridge to nowhere: methane emissions and the greenhouse gas footprint of natural gas